1. Field of the Invention
The present invention relates to separators for winding-type lithium secondary batteries having gel-type polymer electrolytes and a manufacturing method for the same, and more particularly, to separators which can be used for winding-type lithium secondary batteries having gel-type polymer electrolytes prepared by immersing a monomer having good ionic conductivity in an electrolytic solution including a lithium salt and an organic solvent and thermally polymerizing the monomer, and a manufacturing method for the same.
2. Description of the Related Art
In general, a non-aqueous lithium secondary battery includes an anode, a lithium electrolyte prepared from a lithium salt dissolved in at least one organic solvent, and a cathode of an electrochemically active material, which is generally a transition-metal chalcogenide. During discharging, lithium ions generated from the anode release electrical energy, and simultaneously move through a liquid electrolyte to the electrochemically active material of the cathode that takes up the lithium ions. During charging, the flow of lithium ions is reversed so that the lithium ions are released from the cathode active material and returned to the anode through the liquid electrolyte to then be plated on the anode. Non-aqueous lithium secondary batteries are disclosed in U.S. Pat. Nos. 4,472,487, 4,668,595, 5,028,500, 5,441,830, 5,460,904 and 5,540,741, the disclosures of which are incorporated by reference.
To prevent the growth of dendrite and sponge lithium, a metallic lithium anode is replaced with a carbon anode. The carbon anode is made of a carbon material such as cokes or graphite, into which lithium ions are intercalated to form LixC6. During the operation of such batteries, as also occurs in a battery having a metallic lithium anode, lithium ions are released from the carbon anode and move through an electrolyte to the cathode that takes up the lithium ions. During recharging, the lithium ions return to the anode to then be intercalated back into carbon. Since metallic lithium does not exist in the battery, the anode is seldom dissolved even under severe conditions. Also, since the lithium is recombined in the anode by intercalation and not by plating, dendrite or sponge lithium growth does not occur.
In the above-described lithium secondary battery manufactured using a carbon anode and a liquid electrolyte, a porous polyethylene film is typically used as a separator. The porous polyethylene film has a shutdown function at 140° C. and has excellent mechanical strength.
In recent years, lithium secondary batteries using porous polymer matrixes as separators have emerged to prove that the use of a porous polymer matrix can improve the ionic conductivity of a battery. One method of manufacturing the porous polymer matrix includes forming a polymer structure containing a plasticizer, such as dibutylphthalate, and forming pores in the polymer structure by removing the plasticizer. Currently, the plasticizer is usually removed by extraction methods using an organic solvent such as dimethylether, methanol and cyclohexane.
Lithium secondary batteries using such porous polymer matrixes are generally prepared by stacking an anode, a cathode and a porous polymer matrix interposed therebetween, and laminating the stack. However, according to this method, the porous polymer matrix is liable to break during winding. Thus, conventional manufacturing facilities for winding-type lithium ion batteries cannot be used for manufacturing the secondary batteries having a porous matrix.
To overcome this problem, lithium secondary batteries containing a gel-type polymer electrolyte have most recently been manufactured by preparing a jelly-roll type battery precursor comprising an anode, a cathode and a separator interposed therebetween as also done in the conventional lithium ion battery preparation method. A liquid electrolyte and a monomer having good ionic conductivity are added to the jelly-roll type battery precursor and, finally, thermally polymerizing the resultant structure.
The porous polyethylene films that have been used in preparation of conventional lithium ion batteries are still typically being used as separators for the lithium secondary batteries having the gel-type polymer electrolyte. However, the porous polyethylene films are disadvantageously expensive.